1. Field of the Invention
The present invention relates to a four-wheel steering vehicle having a front wheel steering mechanism and a rear wheel steering mechanism, and more particularly to an electric control apparatus for the rear wheel steering mechanism.
2. Discussion of the Prior Art
In recent years, there have been developed various kinds of four-wheel steering vehicles, the rear road wheels of which are steered under control of an electric control apparatus. For example, Japanese Patent Early Publication No. 204180/1990 discloses an electric control apparatus for the four-wheel steering vehicle which is designed to detect a yaw rate of the vehicle body and a steering angle of the front road wheels for determining a steering amount of the rear road wheels in accordance with the detected yaw rate and steering angle. The detection of the yaw rate and the steering angle is conducted to control a steering amount of the rear road wheels in accordance with physical values related to a travelling condition of the vehicle. In usual automotive vehicles, however, the vehicle body is constructed to be deformed during turning travel of the vehicle. Particularly, the long body of a large size truck or bus is greatly deformed during turning travel to cause a lateral displacement between the fore-and-after axes at the front and rear parts of the vehicle body as shown in FIG. 4. As a result, the lateral axis between the rear road wheels may not be maintained in parallel with the lateral axis between the front road wheels. This results in unwanted changes in steering operation of the vehicle.